Temporal plankton dynamics in an oligotrophic maritime Antarctic lake

1 The population density, diversity and productivity of the microbial plankton in an oligotrophic maritime Antarctic lake were studied for a 15-month period between December 1994 and February 1996. 2 In the lake, concentrations of nutrients and dissolved organic carbon were uniformly low, temperature varied over a small annual range of 0.1–3 °C, and the surface was ice-covered except during a period of approximately 6 weeks in summer. 3 The total of 57 morphotypes of protozoa observed during the study is a higher taxonomic diversity than previously reported from continental Antarctic lakes, but lower than that found in more eutrophic maritime Antarctic lakes. Likewise, planktonic abundance and productivity were lower than has been reported in other lakes on Signy Island, but generally higher than those of lakes on the Antarctic continent. 4 There were marked seasonal and interannual variations in planktonic population density. 5 Chlorophyll a concentrations ranged from undetectable to 4.2 µg L-1 and the greatest rate of primary productivity measured was 4.5 mg C m-3 h-1. The phytoplankton was dominated by small chlorophytes and chrysophytes, with phototrophic nanoflagellate abundance ranging from 1.1 × 103 to 1.2 × 107 L-1. 6 Bacterial densities of 3.6 × 108 to 1.9 × 1010 L-1 were recorded and bacterial productivity reached a peak of 0.36 µg C L-1 h-1. Numbers of heterotrophic nanoflagellates between 5.0 × 104 and 1.8 × 107 L-1, and of ciliates from undetectable to 1.1 × 104 L-1 were observed. Naked amoebae were usually rare, but occasionally reached peaks of up to 1.5 × 103 L-1

Variação espacial e temporal dos fatores limnológicos em riachos da microbacia do rio São Francisco Verdadeiro Spatial and temporal variation i of limnological factors in streams of the micro-basin of the São Francisco Verdadeiro river

Com objetivo de caracterizar espacial e temporalmente os riachos da microbacia do rio São Francisco Verdadeiro, Marechal Cândido Rondon - PR, em função de fatores limnológicos, foram estabelecidas seis estações de coleta em três riachos (Curvado, Ajuricaba e Arroio Fundo), sendo uma estação na foz e outra na nascente. As coletas de água foram obtidas entre junho de 2005 e maio de 2006, totalizando 72 amostras. Foram analisadas as seguintes variáveis: temperatura, pH, condutividade elétrica, oxigênio dissolvido, turbidez, material em suspensão, fósforo total dissolvido, ortofosfato, nitrogênio total e nitrogênio amoniacal. De acordo com a análise de variância, houve baixa variabilidade entre as estações de coleta, porém a variação temporal demonstrou a influência do ciclo hidrológico. A análise de componentes principais (ACP) discriminou o sistema com base nos maiores valores registrados para as variáveis limnológicas, ficando separados os períodos influenciados pelas secas e pelas chuvas. A maior parte das variáveis está associada com o período de chuvas, no entanto há concentrações esporádicas de nutrientes na seca e menores valores de oxigênio dissolvido, demonstrando a dificuldade de diluição nesse período, causada pela diminuição da chuva e, consequentemente, do volume de água dos riachos.<br>In order to characterize spatially and temporarily streams of the São Francisco Verdadeiro river micro-basin in Marechal Cândido Rondon, State of Paraná, Brazil, according to limnological factors, six sampling stations were established in three streams (Curvado, Ajuricaba and Arroio Fundo) - one station at the mouth of the river and other at the spring of the streams. Water samples were collected between June 2005 and May 2006, totalizing 72 samples. The following variables were analyzed: temperature, pH, electric conductivity, dissolved oxygen, turbidity, suspended matter, total dissolved phosphorus, orthophosphate, total nitrogen and amoniacal nitrogen. According to ANOVA (variance analysis) it was observed low variability among the sampling stations however the temporal variation demonstrated the influence of the hydrologic cycle. The Principal Components Analysis (PCA) discriminated the system based on the largest values registered for the limnological variables separating the periods influenced by droughts and rains. Most of the variables are associated with the rain period; however, sporadic concentration of nutrients in the drought and smaller dissolved oxygen values indicates a dilution difficulty in this period, caused by decrease of rain and consequently decrease of water volume in the streams